The safety of the blood supply continues to be threatened by the transmission of pathogens by transfusion. While the threat posed by the human immunodeficiency virus (HIV) and the Acquired Immune Deficiency Syndrome (AIDS) is now widely publicized, contamination of blood products with a number of other blood-borne infectious viral agents is of even greater concern. See R. Y. Dodd, In: Transfusion Medicine in the 1990's (American Assoc. Blood Banks 1990) (S. J. Nance, ed.). For example, in the United States, it is estimated that five to ten percent of multiply transfused recipients develop hepatitis accounting for many thousands of cases annually.
The safety of the blood supply cannot be assured by merely testing the blood for pathogens before transfusion. Most testing relies on the detection of antibodies to the pathogen in the prospective blood donor. It is now well-documented that infectious agents can be transmitted by "seronegative" blood donors, i.e., donors that have no detectable antibodies to the pathogen. For example, thirteen cases of transfusion-related AIDS have been reported to the Centers for Disease Control (CDC) among recipients of blood that was pretested and found negative for antibody to the HIV-1 virus.
Most importantly, routine serologic testing will not detect new infectious agents. Each time a new infectious agent is identified, a new test must be designed, developed and approved. During this time, transfusion recipients are at risk. The most dramatic example of this is found in hemophiliac patients receiving repeated exposure to plasma or clotting factor concentrates. While a test for the HIV virus was being developed, this patient population was exposed to untested blood products. As a result, greater than ninety percent of this population now have serologic evidence of past hepatitis or HIV infection, and many have developed overt hepatitis or AIDS.
An alternative approach to eliminate transmission of diseases through blood products is to develop a means to inactivate pathogens in transfusion products. Several methods have been reported to be effective in inactivating or eliminating viral agents, HIV in particular, in human plasma and its derivatives. These methods include thermal inactivation, .gamma. irradiation, laser-UV irradiation, UV irradiation in the presence of .beta.-propiolactone, use of organic solvent and detergent combinations and laser-visible light irradiation in the presence of hematoporphyrin. Unfortunately, most of these methods harm the blood cellular components as well.
A more encouraging approach to blood decontamination is the photochemical decontamination (PCD) process using psoralens. Psoralens are tricyclic compounds formed by the linear fusion of a furan ring with a coumarin. Psoralens can intercalate between the base pairs of double-stranded nucleic acids, forming covalent adducts to pyrimidine bases upon absorption of longwave ultraviolet light (UVA). G. D. Cimino et al., Ann. Rev. Biochem. 54:1151 (1985). Hearst Et al., Quart. Rev. Biophys. 17:1 (1984). If there is a second pyrimidine adjacent to a psoralen-pyrimidine monoadduct and on the opposite strand, absorption of a second photon can lead to formation of a diadduct which functions as an interstrand crosslink. S. T. Isaacs et al., Biochemistry 16:1058 (1977). S. T. Isaacs et al., Trends in Photobiology (Plenum) pp. 279-294 (1982). J. Tessman et al., Biochem. 24:1669 (1985). Hearst et al., U.S. Pat. Nos. 4,124,589, 4,169,204, and 4,196,281, hereby incorporated by reference.
The PCD process has been shown to inactivate a wide range of pathogens in some blood products. See H. J. Alter et al., The Lancet. (ii:1446) (1988). L. Lin et al., Blood 74:517 (1989). G. P. Wiesehahn et al., U.S. Pat. Nos. 4,727,027 and 4,748,120, hereby incorporated by reference, describe the use of PCD process utilizing a combination of 8-methoxypsoralen (S-MOP) and irradiation. They show that the PCD process can effectively inactivate a number of pathogens, including intracellular HIV. Furthermore, the harm to the blood product that would otherwise occur in the PCD process because of energy transfer, was specifically suppressed by limiting the concentration of molecular oxygen.
What remains unclear about the PCD process is the efficiency of inactivation of free viral and provital genetic material, particularly with RNA viruses. Because of the low sensitivity and time consuming nature of current biological culture methods, there is no adequate assay at present for measuring pathogen inactivation in blood products.